Linear and switch-mode voltage regulators constitute fundamental building blocks of today's power management integrated circuits (ICs). In switch mode voltage regulators, the output voltage is almost always soft-started upon enabling the regulator. In linear and low-dropout regulators (LDOs), the requirements usually depend on the application, and can either be implemented as a soft-startup or a fast-startup. One important requirement that can drive the need for soft-starting a given voltage regulator is the prevention of excessive inrush currents resulting from a power-up transient.
In portable device applications which utilize Universal Serial Bus (USB) communications, prevention of excessive inrush currents upon startup can be very important. Specifically, the USB standard can impose very strict requirements regarding an amount of current that can be provided on a USB power bus. As a result, it may be highly desirable to soft-start one or more power regulators on the USB power bus. In addition, regulator controllers that rely on external power devices may require flexible use of a wide range of off-chip power devices having been provided from different manufacturers. Because external power devices can greatly vary with regard to voltage/current (V/I) characteristics, soft-starting may be important to mitigate damage to the external power elements or to mitigate fault conditions due to excessive inrush current.
A variety of soft-start devices have been implemented to mitigate inrush current. However, there is an ever increasing demand for power regulation circuitry and other consumer electronics to operate with increased efficiency and at a reduced size. In addition, there is a current trend of integrating numerous linear, low-dropout, and switching regulators into common power management ICs. Therefore, a soft-starting circuit having a compact and efficient design is desirable to reduce silicon die-are and cost.